Pneumatic refining of pig iron

ABSTRACT

1. A PIG IRON REFINING PROCESS CARRIED OUT IN A CONVERTER HAVING AT LEAST ONE TUYERE COMPRISING AN INNER TUBE WITHIN AN OUTER TUBE, THE PROCESS INCLUDING (A) A FIRST PHASE COMPRISING THE SEQUENTIAL STEPS OF (I) BLOWING A REDUCING FLUID IN THROUGH THE OUTER TUBE, AND SIMULTANEOUSLY BLOWING A SUSPENSION OF LIME IN A NON-OXIDIZING CARRIER GAS THROUGH THE INNER TUBE, WHEREBY A SLAG IS FORMED IN THE CONVERTER; AND (II) REMOVING FROM THE CONVERTER AT LEAST PART OF THE SLAG; AND (B) A SECOND PHASE COMPRISING BLOWING A FLUID CONSISTING OF AN OXYGEN-CONTAINING GAS WHOSE MAJOR COMPONENT IS OXYGEN THROUGH THE INNER TUBE, AND SIMULTANEOUSLY BLOWING A PROTECTIVE FLUID THROUGH THE OUTER TUBE.

United States Patent Int. on. one 7/08 US. Cl. 75-52 11 Claims ABSTRACT OF THE DISCLOSURE A two stage process is carried out in a converter having tuyeres in its lower part. First a suspension of lime in a carrier gas is blown in, and the slag formed is removed. Secondly, an oxygen-containing gas, such as commercial oxygen, is blown in.

The present invention relates to improvements in a process for the pneumatic refining of pig iron in which the refining gas constituted totally or to a large extent by commercially pure oxygen is blown into the refining bath through one or more tuyeres housed in the lower part (preferably in the bottom) of the converter.

It is well known that, in refining processes using blowing through the converter bottom, the oxidizing character of the pig iron refining gases has for a long time been a major obstacle to the longevity of the converter bottom, principally when the refining gas has a high oxygen content.

The use of special tuyeres with two coaxial tubes has nevertheless facilitated the blowing of pure oxygen through the converter bottom without appreciable detriment to the bottom, the oxygen jet emerging from the central tube being surrounded by a protective fluid (i.e. fluid protecting the bottom from the direct action of the oxygen jet) injected via the peripheral conduit defined by the outer tube.

It is also known that, in order to obtain an acceptable final sulphur content in the steel, desulphurization is generally carried out with the help of soda. Thanks to this operation, which is carried out in the ladle prior to the refining process, the result, after addition of scrap and lime into the converter, is a steel containing between 0.020 and 0.030% sulphur, on the basis of pig iron containing, in the mixer, approximately 0.060% sulphur.

The object of the present invention is to provide a refining process in which desulphurizing of the bath takes place in the actual converter.

This pig iron refining process, in which use is made of a converter comprising at least one tuyere with two coaxial tubes housed in the converter bottom or in the lower part of the converter wall, includes two phases; during the first phase of the refining operation a fluid with a reducing character, such as a hydrocarbon, is blown into the molten metal via the peripheral conduit of the tuyere, and simultaneously a preferably non-oxidizing carrier gas which has lime in suspension (preferably in the form of a powder) is blown in via the central conduit of the tuyere; after removal of at least part of the slag formed in the course of the first phase there follows a second phase of blowing into the bath via the central conduit of the tuyere of a gas whose major component is oxygen, the oxygen preferably being commercially pure, while a protective fluid is blown in via the peripheral conduit, preferably of the same nature as the fluid with a reducing character injected via this same peripheral conduit in the course of the first phase.

3,843,354 Patented Oct. 22, 1974 The carrier gas which has lime in suspension and is blown in via the central conduit in the course of the first phase of the operation can be an inert gas (i.e. a gas which is inert to the contents of the converter) such as nitrogen or aron; if necessary, however, the carrier gas can be of the same nature (i.e. composition) as the fluid with a reducing character blown in via the peripheral conduit. A preferential embodiment of the invention consists in the use of natural gas (a hydrocarbon gas obtained from the earths crust). The use as a gas for supporting the lime of a gaseous mixture comprising notably any two of the gases referred to above does not represent a departure from the scope of the invention.

An example of one refining process according to the invention is described below but is not to be taken as limiting the scope of the invention.

A mass of 17.6 tonnes of molten pig iron containing 0.068% sulphur was introduced into a converter having a perforated base fitted with tuyeres with two coaxial tubes whose internal and external diameters were respectively 12 and 15 mm., and 17 and 22 mm.

After clearing the tubes with nitrogen there followed, after 35 seconds, the simultaneous blowing in of 2 m3 of natural gas via the peripheral conduit of the tuyeres and kg. of powdered lime, placed in suspension in 17 m. of natural gas, via the central conduit of the tuyeres. These quantities of natural gas and lime corresponding to consumption of 1.08 m. of natural gas and 5 kg. of powdered lime per tonne of pig iron. The sulphur content of the bath was at this moment 0.010%.

The slagging of the bath by tipping the converter was followed by renewed clearing of the tubes by means of nitrogen. After addition of the usual scrap the converter was raised and pure oxygen and natural gas were blown in respectively via the central conduits and peripheral conduits of the tuyeres until the desired degree of refining had been achieved. The final steel had a sulphur content of 0.015%.

The process described above can be used Without major changes being made to suitable existing installations. It may however be of advantage to use internal tubes of tuyeres Whose inside walls are covered by a special lining, such as an alumina lining, to increase the resistance of this tube to abrasion due to the passage of the powdered lime.

We claim:

1. A pig iron refining process carried out in a converter having at least one tuyere comprising an inner tube within an outer tube, the process including (a) a first phase comprising the sequential steps of (i) blowing a reducing fluid in through the outer tube, and simultaneously blowing a suspension of lime in a non-oxidizing carrier gas through the inner tube, whereby a slag is formed in the converter; and

(ii) removing from the converter at least part of the slag; and

(b) a second phase comprising blowing a fluid consisting of an oxygen-containing gas whose major component is oxygen through the inner tube, and simultaneously blowing a protective fluid through the outer tube.

2. A process as claimed in Claim 1, wherein the lime is in the form of powder.

3. A process as claimed in Claim 1, wherein the protective fluid is a reducing fluid.

4. A process as claimed in Claim 1, wherein the carrier gas is a reducing fluid.

5. A process as claimed in Claim 1, wherein the reducing fluid comprises natural gas.

6, A process as claimed in Claim 1, wherein the carrier gas is gas which is inert to the contents of the converter.

7. A process as claimed in Claim 6, wherein the carrier gas is selected from the group consisting of nitrogen and argon.

8. A process as claimed in Claim 1, wherein the internal surface of the inner tube of the tuyere is of an abrasionresistant material.

9. A process as claimed in Claim 1, wherein the abrasion-resistant material comprises alumina.

10. A process as claimed in Claim 1, wherein the oxygen-containing gas is commercially pure oxygen.

11. A process as claimed in Claim 1, wherein the reducing fluid comprises at least one hydrocarbon.

References Cited UNITED STATES PATENTS 12/ 1972 Knuppel 7560 2/1963 Metz 7552 7/1962 Nelson 7560 5/1960 Klepp 7552 7/1967 Moustier 7560 10/ 1958 Savard 7560 US. Cl. X.R. 

1. A PIG IRON REFINING PROCESS CARRIED OUT IN A CONVERTER HAVING AT LEAST ONE TUYERE COMPRISING AN INNER TUBE WITHIN AN OUTER TUBE, THE PROCESS INCLUDING (A) A FIRST PHASE COMPRISING THE SEQUENTIAL STEPS OF (I) BLOWING A REDUCING FLUID IN THROUGH THE OUTER TUBE, AND SIMULTANEOUSLY BLOWING A SUSPENSION OF LIME IN A NON-OXIDIZING CARRIER GAS THROUGH THE INNER TUBE, WHEREBY A SLAG IS FORMED IN THE CONVERTER; AND (II) REMOVING FROM THE CONVERTER AT LEAST PART OF THE SLAG; AND (B) A SECOND PHASE COMPRISING BLOWING A FLUID CONSISTING OF AN OXYGEN-CONTAINING GAS WHOSE MAJOR COMPONENT IS OXYGEN THROUGH THE INNER TUBE, AND SIMULTANEOUSLY BLOWING A PROTECTIVE FLUID THROUGH THE OUTER TUBE. 